UK consumers get through millions of teabags every day to make their favourite drink yet the vast majority are not fully biodegradable, a consumer organisation warns today.

A report published today by Which? Gardening reveals that teabags produced by top tea manufacturers such as Tetley, PG Tips, Twinnings, Clipper and Typhoo are only between 70-80% biodegradable. As a result, gardeners are finding the net part of teabags - caused by the inclusion of heat-resistant polypropylene - left on their compost heaps.

Which? Gardening contacted the major tea manufacturers to check the content of their products. PG Tips responded: "'Like most of the teabags in the UK, our teabags are made with about 80% paper fibre, which is fully compostable along with the tea leaves contained in the bag. The remaining packaging includes a small amount of plastic which is not fully biodegradable."

Teadirect said: "Our teabags are 70% compostable" while its sustainability manager Whitney Kakos claimed the the use of the plastic polypropylene was "an industry-wide practice."

A spokesperson for Clipper Teas said: "Our teabags are composed of vegetable and wood fibres and are sealed with a low level of polypropylene. Our teabags are suitable for inclusion in compost heaps where the fibres will break down. Any remaining fibres will be small and can be dispersed in the soil."

The government waste body Wrap also advises people to compost teabags even when they contain polypropylene. Lynne Gunn, Wrap's home composting expert, said: "Our advice remains that teabags are suitable for composting. If the bags are still visible when you want to use the compost, they can be sieved out or picked off the surface of the soil. You can also speed up the composting process by ripping open the bags.'"

Harriet Kopinska, home composting project co-ordinator at Garden Organic, said: "We would still tell people to put teabags in their compost, as composting is the better environmental option. But where possible tear the bags first. Even better, use loose tea."

But the UK Tea Council confirmed that we are creatures of habit, with teabags accounting for 96% of the 165m cups of tea drunk every day in the UK.

Which? Gardening said the recyclability of teabags did not seem to be high on manufacturers' agendas, and claimed to have found only one brand of conventional teabag which is polypropylene-free: Jacksons of Piccadilly. Teadirect's Whitney Kakos added: "Most consumers don't notice (the polypropylene) and probably don't care."

Bags which are fully biodegradable include those that are stitched rather than sealed. Another green option are increasingly popular Japanese-style pyramids such as "tea temples" manufactured by teapigs.co.uk, containing whole leaf (as opposed to ground) and arguably better-tasting tea. The company has recently changed its entire range from nylon mesh to a 100% compostable material made from corn starch.